The present invention relates to a three bearing flexible shaft for high speed turbomachinery and, more particularly, a three bearing flexible shaft that improves turbomachine efficiency while mitigating rotor critical speeds.
Conventional aircraft environmental control systems incorporate an air cycle machine, also referred to as an air cycle cooling machine, for use in cooling and dehumidifying air for supply to the aircraft cabin for occupant comfort. Typically, such air cycle machines are three wheel machines comprised of a compressor, a turbine, and a fan disposed at axially spaced intervals along a common shaft, the turbine driving both the compressor and the fan. The three wheels are supported for rotation about the axis of the shaft on one or more bearing assemblies disposed about the drive shaft, such as a pair of spaced bearing assemblies, one bearing assembly disposed intermediate each pair of adjacent spaced wheels. Although the bearing assemblies may be ball bearings or the like, hydrodynamic film bearings, such as gas film foil bearings, are often utilized on air cycle machines.
On aircraft powered by turbine engines, the air to be conditioned in the air cycle machine is typically compressed air bled from one or more of the compressor stages of the turbine engine. In conventional systems, this bleed air is passed through the air cycle machine compressor wherein it is further compressed, thence passed through a condensing heat exchanger to cool the compressed air sufficiently to condense moisture therefrom thereby dehumidifying the air before expanding the dehumidified compressed air in the turbine of the air cycle machine to both extract energy from the compressed air so as to drive the shaft and also to cool the expanded turbine exhaust air for use as the cooling fluid in the aforementioned condensing heat exchanger before it is supplied to the cabin as conditioned cooling air. The compressed bleed air being supplied to the compressor of the air cycle machine is typically precooled by passing it through a precooling heat exchanger in heat exchange relationship with ambient air drawn through the precooling heat exchanger by the fan of the air cycle machine.
Although conventional three wheel air cycle machines are satisfactory for use in such conventional environmental control systems, such three wheel air cycle machines are not suitable for use in the aircraft environmental control system disclosed in U.S. Pat. No. 5,086,622,wherein the expanded and cooled turbine exhaust air which has passed as the cooling fluid in heat exchange relationship with the compressed air passing through the condensing heat exchanger is, after discharge from the cooling fluid path of the condensing heat exchanger, expanded in a second turbine to cool the expanded air to a desired temperature for supply to the cabin as conditioned cooling air and to extract further energy therefrom. To be suitable for use in this improved aircraft air conditioning system, an air cycle machine must have four wheels, i.e. two turbines, a compressor and a fan.
Referring to FIG. 1, four wheel air cycle machines 100 currently use journal bearings 102a, 102b to support a rotating assembly 104. This structure may minimize the spacing between the wheels (such as turbines 106, compressor 108 and fan 110) in order to prevent bending critical speeds from being in the operating speed range. This limitation in the spacing between the wheels, however, may result in sub-optimal fan and/or compressor aerodynamic efficiency.
As can be seen, there is a need for a four wheel air cycle machine design that may allow for improved aerodynamic efficiency while mitigating the bending critical speeds from the operating speed range.